What tools do Volcanologists use to study volcanoes?

Photograph of a tiltmeter courtesy of U.S. Geological Survey.

Volcanologists use many different kinds of tools including instruments that detect and record earthquakes (seismometers and seimographs), instruments that measure ground deformation (EDM, Leveling, GPS, tilt), instruments that detect and measure volcanic gases (COSPEC), instruments that determine how much lava is moving underground (VLF, EM-31), video and still cameras, infrared cameras and cameras that work remotely. This is by no means a complete list, volcanologists will use whatever tool they can to help them understand volcanoes. See if you can think of something you use in everyday life that would help you learn about an active volcano! Volcano Monitoring Techniques has more information about monitoring volcanoes.

Seismometers

This photo shows the earthquake record for the onset of a flank eruption on Kilauea Volcano in November of 1979. Photograph by Robert Decker, U.S. Geological Survey.

Scientists use seismometers to determine the location and strength of earthquakes. The seismometer is the device that actually measures the ground shaking. The signal is transferred to a seismograph which plots the data as a squiggly line called a seismogram. Depending on how far away the earthquake is from the instrument and how strong the earthquake is, the squiggly line can be jagged or barely noticeable. The Richter scale was developed so that earthquakes could be compared in an objective way. For example, a medium-sized earthquake might be really devastating if it occurs near a city while a big earthquake might be go unnoticed if it happens out in the desert. The Richter scale ignores all the "human" effects and allows scientists to determine the amount of energy released by an earthquake. You probably know that it is not a linear scale. For example, a 4 on the Richter scale is not twice as strong as a 2. For each step on the Richter scale there is a multiple of 30x the energy released. Therefore a magnitude 3 releases 30 times as much energy as a magnitude 2. A magnitude 4 releases 30 times as much as a magnitude 3 therefore a magnitude 4 is 30 x 30 as strong as a magnitude 2 or 900 times as strong as a magnitude 2!

The Mercalli scale is a scale that takes into account the human side of earthquakes. It has categories such as "barely felt", "dishes broken" all the way up to "buildings collapsing". Since collapse depends on things like how tall or how well built buildings are, this Mercalli scale is much more subjective.

The best way to determine where a magma chamber is, and how big it is, is to use seismicity. This can be done in two general ways. First it is important to know that when seismic waves encounter molten material they either slow way down or don't pass through at all. One way to locate a magma chamber is to put seismometers all around a volcano and record the seismic waves generated by far-away earthquakes. You don't need the earthquakes to be particularly big. Your ring of seismometers collect data from earthquake waves coming in from all directions. Some of the waves have traveled directly under your volcano. If there is a magma chamber present, then the seismometer on the opposite side of the volcano from where the earthquake originated won't see a seismic signal since the waves were "blocked" by the magma chamber. The seismometers on either side will see the waves from that particular earthquake since they will have passed on either side of the magma chamber.

The other technique is to monitor the seismicity generated by the volcano itself. Magma chambers often swell and contract, but again the small earthquakes associated with this activity cannot pass through the magma chamber itself so a detailed seismic map will show a "shadow zone" where the magma chamber is.

Other monitoring techniques involve observing how the volcano swells prior to an eruption using EDM(Electronic Distance Measurement), surveying instruments and GPS. If you measure enough places on the surface, you can plug all the data into computer programs that can estimate how deep the magma chamber is, how big it is, and where new magma is.

These techniques have all been perfected at the Hawaiian Volcano Observatory, and are now used all over the world.

Scott Rowland

GPS

This photo shows a GPS receiver on the south flank of Kilauea. Photo by Steve Mattox, March 1992.

GPS(Global Positioning System) is a method to determine horizontal and vertical position of locations on Earth. It uses satellites that broadcast a signal and receivers that pick up and record the signal. Hikers can use a hand-held GPS to find their position. Farmers here in North Dakota use GPS to locate specific areas in their fields that need special attention. Volcanologist use GPS as a powerful tool to detect movement at shallow depths beneath the surface of volcanoes.

GPS uses the relationship between velocity, distance, and time (velocity equals distance divided by time). With GPS, the velocity is the speed of light. The signal from the satellite includes the time it was sent. The receiver notes the time the signal arrived. The difference is the amount of time the signal was in transit. Knowing the velocity and time (and the position of the satellite) allows the distance to be calculated.

Volcanologists use relative position, the vertical and horizontal changes of the same location over a period of time. For example, as magma moves up into a volcano the volcano swells and distances between points on the volcano increases (think of dots on a balloon) and the elevation at specific points increases. Such a change is a common precursor to an eruption.

GPS is used on the Hawaiian volcanoes and several volcanoes in the Cascades. It has also been used on volcanoes in Iceland, Italy, and Japan.

For more information on GPS read John Dvorak article in Earthquakes & Volcanoes (1992, v. 23).

Steve Mattox